Adsorption and Physical Property Methods

The analysis of the viscous fractions of petroleum by physical property correlations, and other short cut methods, is a very fascinating one. This approach is very different from the complete separation used by API Research Project 6 (19). The American Petroleum Inst. project gives us basic information which can be used to check the rapid correlative methods. This paper briefly reviews the available rapid correlative methods for obtaining the composition of petroleum oils and provides a comparison showing how these methods work out when used on a group of 44 oils for which thorough analyses are available. These thorough analyses involve separation of the oil into its aromatic and saturated components by adsorption, and application of well-established correlations to the determination of the composition of the fractions. In recent years there has been a rapid increase in the use of carbon-type composition to describe the average composition of lubricating oil (8, 18). Carbon-type analysis reports the percentage of the total number of carbon atoms that occur in aromatic rings, per cent CA, naphthene rings, per cent CN, and paraffin chains, both free and combined, per cent CP. For example, Fig. 1 shows what may be considered a typical saturated lubricating oil molecule. It contains 31 carbon atoms, 13 of which are in naphthene rings. Therefore, per cent CN equals 42. It contains 18 carbons in paraffin chain structures. Therefore, per cent CP equals 58. Figure 2 shows a typical aromatic molecule which contains a total of 31 carbon atoms, including three aromatic rings and two naphthene rings. There are 14 carbons in aromatic structures; per cent CA therefore equals 45. There are 7 carbons in naphthene structures; per cent CN therefore equals 23. There are 10 carbons in paraffin chain structures; per cent CP therefore equals 32.